The integrity of a well bore penetrating a subterranean formation is enhanced by joining individual lengths of relatively large diameter metal tubulars together, which are termed casing, to form a casing string, which is positioned in the well bore. The casing string is commonly cemented to the well bore face and subsequently perforated at the production interval of the well bore by detonating shaped explosive charges therein. The resulting perforations extend through the casing and cement a short distance into the formation. In addition to increasing the integrity of the well bore, the perforated casing string provides a conduit for producing fluids through the well bore to the surface.
In certain instances it is desirable to conduct perforating operations while maintaining the well bore pressure in an overbalanced condition with respect to the formation pressure. The overbalanced well bore pressure typically exceeds the formation fracturing pressure which induces hydraulic fracturing in the vicinity of the perforations. Such deliberate inducement of fractures in the formation at the perforations is generally termed stimulation. While the perforations often extend only a matter of inches into the formation, a fracture network may extend several feet into the formation. The fracture network provides an enlarged conduit for producing fluids from the formation into the well bore and may significantly increase well productivity.
Gas generating propellants have been utilized in lieu of hydraulic fracturing as an alternate stimulation technique for creating and propagating fractures in a subterranean formation. In accordance with conventional propellant stimulation techniques, a propellant is ignited locally to generate a gas which pressurizes the production interval of the well bore either in association with the perforating step or after the perforating step. The resulting gas creates and propagates fractures in the formation at the production interval of the well bore.
A conventional propellant stimulation tool consists of a propellant body cast from a solid rocket propellant material and an ignition system which includes a starter assembly and an ignition propagator connected to the starter assembly. The starter assembly typically includes a detonator and the ignition propagator is typically a detonator cord. The ignition propagator can optionally include a thin walled aluminum or cardboard sleeve around the detonator cord, which facilitates placement of the detonator cord within the tool.
It has been found that the intrusion of well bore fluids into the ignition system, for example, into the connection between the starter assembly and ignition propagator, can diminish the functionality of the ignition system. Even if a sleeve is provided for the detonator cord, the sleeve is open ended and lacks sufficient structural integrity to effectively seal the ignition system against fluid intrusion therein from the surrounding environment. A common technique for reducing contact between the ignition system and well bore fluids is to wrap the connection between the starter assembly and ignition propagator in a fluid resistant tape.
In any case, the above-described propellant stimulation tool is not universally suited for use in all types of well bores because the tool lacks sufficient mechanical strength to withstand excessive forces encountered in many types of well bores. For example, the present propellant stimulation tool is generally unsuitable for use in small diameter well bores, well bores which are deviated, and/or well bores where the temperature exceeds about 275° F. due to excessive forces therein.
The structural integrity of the above-described propellant stimulation tool can be increased by inserting the propellant body into a reusable metal carrier, which supports the propellant body during placement in the well bore and subsequent ignition of the propellant. Alternatively, the size of the propellant body can be expanded and fitted around a reusable metal carrier which supports the propellant body. U.S. Pat. No. 6,082,450, which is incorporated herein by reference, discloses such a propellant stimulation tool wherein a reusable metal carrier internal to a propellant body supports the propellant body. The propellant stimulation tool of U.S. Pat. No. 6,082,450 advantageously has utility in well bores of varying diameters and orientations. The supported propellant stimulation tool generally provides a repeatable and reliable propellant burn in a discrete or controlled pattern upon ignition of the propellant.
Despite its advantageous performance features, propellant stimulation tool of U.S. Pat. No. 6,082,450, does not fully seal the interior of the tool to well bore fluids. The tool permits the flow or seepage of well bore fluids into the interior of the tool where the fluids can contact the ignition system. Like other prior art tools, a tape wrapping is relied upon to minimize contact between the ignition system and the well bore fluids. Unfortunately, the tape wrapping does not always adequately isolate the ignition system from the well bore fluids. When the tape wrapping fails to satisfactorily protect the ignition system from the well bore fluids, the ability of the detonator cord to properly propagate detonation of the propellant is compromised, which diminishes the repeatability and reliability of the propellant burn and correspondingly diminishes the overall performance of the propellant stimulation tool. As such, a propellant stimulation tool is needed which maintains the ignition system sufficiently dry across a broad range of well bore conditions.
Thus, it is an object of the present invention to provide a stimulation tool for a subterranean formation utilizing a combustion material, such as a propellant, ignited by an ignition system, wherein the tool maintains the ignition system in essential fluid isolation from well bore fluids. It is another object of the present invention to provide a stimulation tool for a subterranean formation utilizing a combustion material ignited by an ignition system, wherein the combustion material is in the form of a solid combustion body maintained on a mounting frame, which also supports an ignition propagator, within the tool. It is still another object of the present invention to provide such a stimulation tool, wherein ignition of a combustion material is carefully controlled by appropriately specifying certain physical parameters of the tool to achieve a substantially reliable and repeatable burn of the combustion material.
These objects and others are accomplished in accordance with the invention described hereafter.